Backing absorbers for ultrasonic transducers are typically comprised of metal particles and other binder composites. U.S. Pat. Nos. 3,973,152, 4,090,153, 4,582,680, and 6,814,618 describe such prior art backing absorbers. U.S. Pat. No. 3,973,152 describes a pressure applied to a multilayer metallic foil that performs as an absorber. However, such structures and techniques are deficient in several aspects. For example, ultrasonic waves do not propagate through relatively small gaps (e.g. gaps on the order of about 0.01 micrometer (um) or greater) between surfaces. Rather, ultrasonic waves are transmitted only through the small areas where the metal layers actually contact or are fused to one another.
Because the metal surface is not ideally flat and microscopic roughness exists, the actual or real contacting area represents a small fraction of the total surface area, and ultrasonic waves propagate through mostly in these small spots where absorption of acoustic waves takes place. This is the mechanism of attenuation of ultrasonic waves in pressurized multiple layers of metal foils. In order to cause the metal foils to be in substantially uniform contact without the aforementioned relatively small gaps, high pressure (e.g. about 50,000 psi (350 MPa) or more) has to be applied to permit acoustic waves to go through most of the boundary area. However, such a structure does not provide appropriate absorption. Therefore, the pressure has to be at a certain value which yields multiple spots of contact thereby providing appropriate attenuation to the waves. However, it is difficult to control the application of pressure in a constant and reproducible manner within this environment. For example, when applying high pressure, metal is usually fatigued and pressure decreases in time, thereby causing the absorption to decrease over time.
A further problem with the known multilayer backing absorber concerns the difficulty in designing the pressurizing structure. Piezoelectric materials such as PZT or crystal are brittle and easily broken by the applied pressure, and yet multiple layers of metallic foils have to be pressed against the piezoelectric layer. This requires that the piezoelectric material hold the pressure. If only the periphery of the multi layer foil is pressurized and the main central region is bonded to piezoelectric material, appropriate pressure cannot appear on each boundary of the multi layer structure. It is difficult to design such a structure, particularly when the size of the piezoelectric layer is thin (less than 0.5 mm) and large (more than 5 mm). Furthermore, the pressurizing structure, which typically includes screws and a holder, make the device bulky. Still further, the absorption and impedance cannot simply be designed to a specified value.
Backing absorbers are relatively difficult to manufacture and control the absorption and acoustic impedance of these devices. Many absorbers are comprised of heavy metal particles mixed with epoxy or polymer as a binder. The density difference makes sediment and thus requires thorough mixing. Moreover, casting must occur immediately after mixing to place the absorber in the desired shape. Such processes are difficult to control. Furthermore, mixing with correct ratios requires accurate weight measurements.
Such problems of difficulty in design, reproducibility and reliability are commonly seen for any absorber including the aforementioned examples. Alternative absorber structures and methods of making absorber structures are desired.